Currently, there is huge demand for converged mobile devices which combine data and telephony capabilities. Technological advances such as extremely low power consumption, improvements in form factor, pricing and co-existence technology for 802.11 (WLAN) and Bluetooth are fueling the demand.
Wireless communication devices such as WLAN and Bluetooth devices are generally constrained to operate in a certain frequency band of the electromagnetic spectrum. The use of frequency bands is licensed by government regulatory agencies, for example, the U.S. Federal Communications Commission (FCC) and the European Radio Communications Office. Licensing is necessary in order to prevent interference between multiple broadcasters trying to use the same frequency band in an area.
Regulatory agencies also specify frequency bands for devices that emit radio frequencies, where licensing is not required. Wireless communication devices using these unlicensed frequency bands generally transmit at low power over a small area. The Industrial, Scientific, or Medical equipment (ISM) band is one such frequency band located between 2.4 and 2.5 GHz. This 2.4 GHz band is used by many wireless communication devices for data and/or voice communication networks.
An example of such a network is defined by the Bluetooth specification. Bluetooth specifies communication protocols for low cost, low power wireless devices that operate over a very small area, the so-called, personal area network. These wireless devices may include, for example, telephone headsets, cell phones, Internet access devices, personal digital assistants, laptop computers, etc. The Bluetooth specification effectively replaces cables used to connect communicating devices, for example, a cell phone and a headset, with a wireless radio link to provide greater ease of use by reducing the tangle of wires frequently associated with personal communication systems. Several such personal communication devices may be wirelessly linked together by using the Bluetooth specification, which derives its name from Harald Blatand (Blatand is Danish for Bluetooth), a 10th century Viking king who united Denmark and Norway.
Bluetooth is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices such as mobile phones, printers, PCs, laptops, and other digital equipment, over a secure, globally unlicensed short-range radio frequency (RF).
Bluetooth is a radio standard and communications protocol primarily designed for low power consumption, with a short range based on low-cost transceiver integrated circuits (ICs) in each device. Bluetooth networks enable these devices to communicate with each other when they are in range.
Bluetooth capability is increasingly built-in to many new products such as phones, printers, modems and headsets. Bluetooth is appropriate for situations when two or more devices are in proximity to each other and do not require high bandwidth. Bluetooth is most commonly used with phones and hand-held computing devices, either using a Bluetooth headset or transferring files from phones/PDAs to computers.
Bluetooth also simplified the discovery and setup of services, in contrast to WLAN which is more analogous to a traditional Ethernet network and requires configuration to set up shared resources, transmit files, set up audio links (e.g., headsets and hands-free devices), whereas Bluetooth devices advertise all the services they provide; thus making the service more accessible, without the need to worry about network addresses, permissions, etc.
Because Bluetooth devices operate in the unlicensed 2.4 GHz RF band, they are subject to radio interference from other wireless devices operating in the same frequency band. To avoid RF interference, the Bluetooth specification divides the 2.4 to 2.5 GHz frequency band into 1 MHz-spaced channels. Each channel signals data packets at 1 Mb/s, using a Gaussian Frequency Shift Keying modulation scheme. A Bluetooth device transmits a modulated data packet to another Bluetooth device for reception. After a data packet is transmitted and received, both devices retune their radio to a different 1 MHz channel, effectively hopping from radio channel to radio channel, i.e., frequency-hopping spread spectrum (FHSS) modulation, within the 2.4 to 2.5 GHz frequency band. In this way, Bluetooth devices use most of the available 2.4 to 2.5 GHz frequency band and if a particular signal packet transmission/reception is compromised by interference on one channel, a subsequent retransmission of the particular signal packet on a different channel is likely to be effective.
Bluetooth devices operate in one of two modes: as a Master device or a Slave device. The Master device provides a network clock and determines the frequency hopping sequence. One or more Slave devices synchronize to the Master's clock and follow the Master's hopping frequency.
Bluetooth is a time division multiplexed system, where the basic unit of operation is a time slot of 625 microsecond duration. The Master device first transmits to the Slave device during a first time slot of 625 microseconds with both devices tuned to the same RF channel. Thus, the Master device transmits and the Slave device receives during the first time slot. Following the first time slot, the two devices retune their radios, or hop, to the next channel in the frequency hopping sequence for the second time slot. During the second time slot, the Slave device must respond whether it successfully understood, or not, the last packet transmitted by the Master during the first time slot. The Slave device thus transmits and the Master device receives during the second time slot. As a Slave device must respond to a Master's transmission, communication between the two devices requires at a minimum two time slots or 1.25 milliseconds.
Data packets, when transmitted over networks, are frequently susceptible to delays by, for example, retransmissions of packets caused by errors, sequence disorders caused by alternative transmission pathways, etc. Packet delays do not cause much of a problem with the transmission of digital data because the digital data may be retransmitted or re-sequenced by the receiver without effecting the operation of computer programs using the digital data. Packet delays or dropped packets during the transmission of voice signals, however, can cause unacceptable quality of service.
The Bluetooth specification version 1.1 provides a Synchronous Connection Oriented (SCO) link for voice packets that is a symmetric link between Master and Slave devices with periodic exchange of voice packets during reserved time slots. The Master device transmits SCO packets to the Slave device at regular intervals, defined as the SCO interval or TSCO, which is counted in time slots. Bandwidth limitations limit the Bluetooth specification to a maximum of three SCO links. Therefore, the widest possible spacing for an SCO pair of time slots, which are sometimes called a voice slot, is every third voice slot. Bluetooth specification version 1.2 provides enhanced SCO links, i.e. eSCO links, which have a larger voice slot size, based on N*625 microsecond time slots, with larger and configurable intervals between voice slots. These eSCO links can be used for both voice or data applications.